Hello, your method for doing this is incorrect. Like Vladimir says, you should use Sphere primitive. In Alias - Sphere/Shell no poll, then you may unstich it to get a square surface, and then you need to adjust the correct position! I also had a similar problem, when I was modelled a cap of a speakers.But finally I've received a perfect result.
This is the project I've mentioned www.behance.net/gallery/PraGma/10681017